The continuation of a research program concerned with the photochemical and photobiological properties of urocanic acid (UA) is outlined. This program is highly relevant to the area of cutaneous photobiology, since UA (a histidine metabolite) accumulates in the skin, and is responsible for appreciable absorption of uv light by the skin. Our long term goals are to completely delineate the unimolecular and bimolecular UA reactions initiated by light. The specific aims of the present proposal include a greater understanding of the photochemical reaction(s) of UA with DNA and testing for cytotoxicity and mutagenesis in bacterial and mammalian cells. The program also outlines exploratory studies of the photochemical reactions of UA with other biological substrates (cholesterol and protein), and the preparation and examination of both synthetic and natural UA analogs.